Although stem cells play a central role in maintaining long-term hematopoiesis, very little is known about the mechanisms controlling proliferation and differentiation of these cells. The aims of this proposal are to gain a better understanding of the mechanisms that influence stem cell development both in vivo and in vitro. A number of methods will be employed to identify stem cells and analyze the effects of various manipulations on this population. Stem cells will be uniquely marked using retroviral vectors, allowing the investigator to follow the fate of individual cells in multiple generations of reconstituted mice. Competitive repopulation assays allow the investigator to analyze the stem cell population in a quantitative fashion, making it possible to estimate the number of stem cells in the original inoculum. Using these approaches, questions concerning the kinetics of stem cell development, characterization of factors that influence their proliferation, and the size of this population in vivo will be addressed. In parallel, the effects of various growth factors and stromal cell populations on the survival and growth of cells in culture will be carried out. Both unfractionated marrow and populations highly enriched for stem cells, using physical separations as well as antibodies, will be used in these analyses.